The present invention relates to a projection data generating method, a pixel data generating method, and a multi-row detector X-ray CT (computed tomography) apparatus, and more particularly to a method of generating projection data for use in producing an image having a large slice thickness, based on raw data collected by an axial scan or helical scan using a multi-row detector, a method of generating pixel data for the image having a large slice thickness, and a multi-row detector X-ray CT apparatus.
FIG. 14 is a flow chart showing pixel data generating processing in a conventional multi-row detector X-ray CT apparatus.
At Step J1, raw data is collected while rotating an X-ray tube and a multi-row detector around a subject to be imaged.
At Step J2, raw data d1 and d2 corresponding to a pixel g in a reconstruction field P are obtained as shown in FIG. 15. Specifically, the raw data d1 and d2 are obtained from two detector rows that lie closest to a point at which a straight line Lg passing through a focal spot of an X-ray tube 21 and the pixel g in the reconstruction field P intersects a multi-row detector 24. The raw data d1 and d2 are then multiplied by cone-beam reconstruction weights Wa and Wb and Z-filter weights wa and wb and are added to calculate a projection datum Dg corresponding to the pixel g in the reconstruction field P.
The cone-beam reconstruction weight Wa is defined as (r1/r0)2, where the distance from the focal spot of the X-ray tube 21 to a detector row j, channel i of the multi-row detector 24 that corresponds to the raw data d1 is represented as r0, and the distance from the focal spot of the X-ray tube 21 to the pixel g in the reconstruction field corresponding to the raw data d1 is represented as r1.
The cone-beam reconstruction weight Wb is similarly defined.
The Z-filter weights wa and wb are defined as wa=b/(a+b) and wb=a/(a+b), where the distance between the point at which the straight line Lg intersects the multi-row detector 24 and a point corresponding to the raw data d1 as measured in the Z-direction of the detector is represented as a, and the distance between the point at which the straight line Lg intersects the multi-row detector 24 and a point corresponding to the raw data d2 as measured in the Z-direction of the detector is represented as b. These are weights as delineated by a Z-filter Fz shown in FIG. 16.
Referring again to FIG. 14, at Step J3, backprojection processing is applied to the projection datum Dg to calculate a pixel datum Gg.
An image obtained by the conventional multi-row detector X-ray CT apparatus has a slice thickness approximately equal to the size of the detector as measured in the Z-axis direction.
There has been a problem that an image cannot be obtained with a large slice thickness, such as a thickness twice or three times the size of the detector as measured in the Z-axis direction.